Hartwig Steckel

Extrusion/spheronization is a common pellet production technique. Typically, microcrystalline cellulose (MCC) is used for this purpose, but this has distinct disadvantages, such as lack of disintegration and a limited capability for controlled drug release. A promising alternative extrusion filler and binder is chitosan, a natural, cationic polymer, which is widely available.  

In this study, the effect of the degree of deacetylation of different chitosan grades on process and pellet properties was investigated. Effects on the drug release (budesonide was used as the model drug) from chitosan pellets were observed. 

It was found that the degree of deacetylation of chitosan markedly influences the process capability of chitosan and, as a result, the pellet-forming properties. The higher the degree of deacetylation, the more stable the process and the better the resulting pellet properties (measured as pellet size, aspect ratio, crushing strength and friability). Oscillatory rheological measurements of the extrudate indicated that a higher degree of deacetylation also influenced the viscoelastic properties of the wet mass. A greater degree of deacetylation clearly showed a higher phase angle and, accordingly, a higher proportion of the viscous modulus.  

These findings correlated well with better extrudability as a result of a higher degree of deacetylation. Drug release was found to be less influenced by the degree of deacetylation. In all cases, a linear drug release from the chitosan pellets was observed. In conclusion, the study underlined that chitosan offers an excellent alternative to MCC for extrusion/spheronization, and allows for a controlled drug release with zero order release kinetics. 

Chitosan (Figure 1) is produced by N-deacetylation of chitin, the second most common natural polymer. This process leads to a cationic polymer that is characterized by pH-dependent solubility in water. In contrast to many other high molecular weight polymers of anionic character, chitosan is only soluble in acidic aqueous solutions. 

Chitosan is of pharmaceutical interest because of its positive attributes with respect to toxicity, biocompatibility and bioavailability.

  In addition to wound-healing effects, chitosan is also said to have fat and cholesterol-binding properties, as well as antiulcerogenic effects. As a result, chitosan has been used for numerous delivery systems, especially in the form of tablets, microparticles, pellets and beads for oral application.

The intention of pellet production is the development of a multiparticulate delivery system, in particular, sustained release formulations for oral application because of their biopharmaceutical advantages compared with monolithic dosage forms. 

Gastric residence time of monoliths, such as nondisintegrating tablets, is highly variable and depends on stomach volume, size and density of chime and particle size. It is also well-accepted that particles smaller than 2–3 mm

(e.g., pellets) rapidly pass the pylorus with constant rate.

				  In addition, gastrointestinal (GI) irritations are reduced by a rapid and homogeneous distribution of pellets in the intestine. A burst effect has so far not been noticed with multiparticulate systems.  

From a technological view, pellets can be favourably used instead of monolithic systems in case incompatible drugs have to be processed. They can be pelletized separately and mixed later, or pellets with different release mechanisms can be mixed individually to give a new modified release profile. Until now, MCC, despite intensive research on alternative materials, is mostly used as a universal binder and filler for extrusion/spheronization. Chitosan represents an alternative filler and binder, and may be a substitute for MCC. Compared with MCC, it is also of natural origin and biodegradable, and so has been widely used in the pharmaceutical and cosmetic industries. Furthermore, chitosan can be degraded by the colonic microflora and could, therefore, be particularly useful for colon-targeted delivery of drugs incorporated into enteric-coated pellets. In addition, it is assumed that the drug release from chitosan pellets differs from that of MCC pellets, so it offers new opportunities for drug delivery. 

Up until now, several techniques have been used to prepare chitosan pellets. A method to produce chitosan beads is the precipitation in an acetic medium by adding a salt-containing solution.

  For the production of cryopellets, Hofmann

 used a chitosan solution (chitosan dispersion) that had been added to a cooled nonsolvent.

Another approach to prepare heterogeneous pellets is to coat cores with a chitosan solution, thus obtaining a release-controlling polymer film.

  In addition, it is possible to produce homogeneous pellets using extrusion/spheronization. For that purpose, Tapia

. dissolved chitosan in diluted acetic acid and then added this as granulation liquid to the powder mixture.

  The wet mass was passed through a ram extruder and spheronized into pellets. The chitosan fraction in the obtained pellets was as low as 2–3% w/w. Chatchawalsaisin

 (using a screw extruder) achieved an increase of the chitosan fraction to 16% in the pellet matrix.

A further increase of the chitosan fraction in the pellets was obtained by Goskonda 

by an extrusion/spheronization technique.

				  For this purpose they used chitosan with different molecular weights and, for the extrusion process, mixed them with colloidal MCC — a spray-dried mixture of 89% MCC and 11% carboxymethylcellulose sodium. However, the amount of chitosan in the obtained pellets did not exceed 40% of the total solid content. In contrast to the study of Goskonda 

, Steckel and Mindermann-Nogly produced MCC pellets with a maximum chitosan fraction of 50% with an extrusion/spheronization technique using demineralized water as the granulation liquid. Substitution of the demineralized water through diluted acetic acid (as the granulation liquid) enabled the process to be performed with pure chitosan. This made the production of pellets with good morphological and mechanical properties without using any further additives possible.

The chitosan used in those studies had a degree of deacetylation greater than 95% and could be handled without any problem. However, subsequent batches of the same specified quality showed inadequate plasticity for the extrusion process. Minor differences in the degree of deacetylation were expected to cause these process differences. Therefore, the aim of this study is to investigate the influence of the degree of deacetylation of chitosan on the production of pure chitosan pellets, as well as on the physicochemical pellet and release properties of the obtained pellets. 

Chitosan (Chitoclear FG 95; Primex, Iceland) with a specified degree of deacetylation (DD) of >95% was used as matrix forming material. Different degrees of deacetylation, as determined by the manufacturer, were used to study the influence of the degree of deacetylation on the extrusion/spheronization properties. The following batches were used: 			

Concentrated acetic acid (Merck, Germany) and demineralized water were mixed and used as the granulation fluid for the extrusion step. The used drug, budesonide, was a gift from AstraZeneca (Germany). All other reagents used (Merck) and solvents were of analytical grade. The water used was either of demineralized or double-distilled quality, depending on the intended use. 

Initially, the drug/chitosan mixtures were blended for 5 min in a Turbula T10 (W.A. Bachhofen AG, Switzerland), deagglomerated by the use of a sieve (250 μm [Retsch GmbH, Germany]) and blended again for 25 min in a Turbula blender. The intermediate sieving step was included to obtain a homogeneous distribution of the drug substance, which is present in a very low concentration. Afterwards, the powder mixture was fed by means of a screw feeder (K-Tron Schweiz AG, Switzerland) to the extruder at a rate of 15 g/min. The extrusion of the different powder mixtures was performed by a power consumption-controlled twin screw extruder (8 ZE 25; Berstorff, Germany) at a fixed power consumption level of 120 W. A detailed description of the functional principle is given by Lindner 

 As a granulation liquid, 0.2 N acetic acid was used as determined in previous studies.

 400–600 g of the extrudate was collected per batch and spheronized in a spheronizer (Type S-320; Nica, Sweden) with a cross-hatched plate at 800 rpm for 5 min. 

The obtained pellets were then dried in a fluid-bed dryer (TR2; Glatt GmbH, Germany) for 30 min at 55 °C. 

Three samples of approximately 8 g were taken during the extrusion and dried at 75 °C during 36 h in a hot air oven (Heraeus T6; Kendro, Germany). The moisture content (

) of the extrudate was calculated according to Equation 1, where 

A sample of approximately 8–10 g was collected during the extrusion process and measured using a plate–plate viscosimeter (Bohlin Instruments, UK) during a fixed period of 120 s with constant split amplitude. The calculation of the phase angle and the proportion of elastic to viscous parts was performed with standard computer software (Bohlin Instruments). Measurements of each batch was performed in triplicate. 

The fine content was removed by using a 500 μm sieve. The shape and size of the pellets >500 μm were determined individually by an image analysis system (Leco 2001; Leco, Germany). The average Feret diameter was calculated indirectly from the (directly calculated) middle Feret diameter as the mean of eight measured Feret diameters from eight different angles of each single pellet. At least 500 pellets/batch were determined. For the evaluation of the pellet shape, the aspect ratio was determined. A detailed description of the test conditions used for pellet analysis is given by Lindner 

To evaluate the shape and surface characteristics, photographs of pellet samples from the different batches were taken by using a scanning electron microscope (Phillips XL120; Phillips, The Netherlands). Pellets were fixed on an aluminium stub with conductive double-sided adhesive tape (Leit-Tabs; Plano GmbH, Germany) and coated with gold in an argon atmosphere (50 Pa) at 50 mA for 50 s (Sputter Coater; Bal-Tec AG, Liechtenstein). 

The porosity (ε) of the pellets was calculated using Equation 2, with ρ

The true density was determined by means of a helium gas pycnometer (Pycnomatic ATC; ThermoFinnigan, USA) and the apparent density was measured using a mercury intrusion porosimeter (Pascal 140; Carlo Erba Instruments, Italy). All measurements were performed in triplicate. 

For the investigation of crushing strength, the pellets were initially conditioned in a climate chamber with a relative humidity of 55% during 24 h to reduce the influence of changing ambient humidity. Afterwards, 50 pellets/batch from the sieve fraction 900–1120 μm were analysed with a texture analyser TAXT2 (Stable Micro Systems, UK). Briefly, a single pellet was centred below the punch. Punch movement down onto the pellet was standardized to 1 mm/s. After contact, the strain was 50% of the height while measuring the force. The arithmetic mean of these 50 measurements was used as the crushing strength in the study. 

For this part of the investigation, a friability tester was used.

) pellets with a size >710 μm were taken and then fluidized by an air stream of 450 L/min for 16 min. The removed pellets were weighed after the test (

The drug release of budesonide-containing pellets was determined by the liberation cell equipment Dissotest CE 6 (Sotax, Switzerland). The drug release medium phosphate buffer pH 7.4 (Ph. Eur.) was degassed in an ultrasonic bath. A pellet mass corresponding to a single dose of 3 mg budesonide was filled into each of the release cells. The release medium was then pumped through the cells at a flow rate of 8 mL/min. During the drug release test, samples were collected out of the discharged medium in discontinuous intervals of 1 min. 100 μL of the samples were taken for HPLC analysis. The HPLC system consisted of a Gynkotek High Precision Pump Model 300 (Gynkotek, Germany), a Kontron HPLC Autosampler 360, a Kontron HPLC Detector 430 (Kontron Instruments, Italy) and LiChrospher 100 RP18 columns (5 μm; 125 mm; [Merck]). The peak integration (wavelength 246 nm) was performed using a computer controlled software (Data System 450; Kontron Instruments). A mobile phase of acetonitrile/phosphate buffer pH 3.0 (Ph. Eur.) mixture (55:45) was used. The amount of drug was calculated using an external standard. Three samples per batch were determined in duplicate by HPLC. 

The extrusion of chitosan with a degree of deacetylation of >99% led to a stable process at a fixed power consumption level of 120 W (Table 1). Other chitosan properties made it necessary to adjust the power consumption to a higher level. The extrusion of chitosan with a degree of deacetylation of 90% at a power consumption level of 120 W resulted in a lack of granulation liquid in the wet powder mass. 

Table 1 Extrusion parameters for the different chitosan batches. 			

Consequently, the mass accumulated in front of the die plate and blocked the holes; therefore, the extrusion had to be stopped. As can be seen in Table 1, indicated by the low standard deviation (SD) of the power consumption, a stable extrusion process was only possible at a 99.9% degree of deacetylation. With a decreasing degree of deacetylation of the used chitosan, the extrusion process became unstable.  

Chitosan, with a 90% degree of deacetylation could not be extruded anymore. The process is unstable because of incomplete wetting and feeding of the wet mass, leading to a blockage of the die plate at irregular intervals. Accordingly, the pressure at the die plate, as well as the power consumption, increased.  

In case the pressure exceeded the binding strength of the extrudate mass, the die holes were pressed free by the accumulated mass in front of the plate and so a drug-containing batch was not produced with this chitosan batch.  

The process parameters of the produced budesonide-containing batches compare favourably to the pure chitosan batches. The influence of the low budesonide fraction in the powder mixture on the extrusion process was, therefore, excluded (Table 1). 

In Figure 2 the moisture content of the different chitosan and chitosan/budesonide batches is shown. It can be seen that the moisture content increases with a decreasing degree of deacetylation — caused by the functional principle of the extruder. Through the mechanism of negative feedback, it met with increasing resistance at the die plate. This led to a higher power increase, which, in turn, increased the volume of granulation fluid needed to return to the fixed power level again. 

Articles

The extrusion and spheronization of chitosan

The lung, with its high surface area, is a physiological organ of interest for the non-invasive application of drugs - both in the treatment of local diseases such as asthma and chronic obstructive pulmonary (COPD) disease, and for the systemic delivery of drugs such as insulin and morphine. Different application devices, including nebulizers, metered-dose inhalers (MDI) and dry powder inhalers (DPI), are used to deliver drugs to the lungs. The phase-out of chlorofluorocarbon (CFC) propellants and the difficulties in reformulating MDIs with new hydrofluroalkane (HFA) propellants have created a growing interest in using DPIs as delivery devices for powdered medication.

As low doses are typically needed for the oral inhalation of drugs, a physiologically acceptable, non-toxic carrier is used to dilute the drug and improve the powder flowability and, consequently, the dosing accuracy. α-lactose monohydrate is a well-known and widely used carrier for DPI applications.

 The efficiency of an inhaler is largely dependent upon the drug and drug particle size in use,

 the type of device used to deliver the powdered material

 and the quality of lactose. Different types and qualities of lactose may influence the performance of a DPI,

 and the lactose quality needs to be carefully selected.	

In this article, the effect of different lactose qualities on a capsule-based, multidose DPI (FlowCaps; Hovione SA, Portugal), has been analysed. The DPI consists of 6 plastic components and several metal parts (Figure 1). Prefilled capsules are stored in the capsule reservoir, one of which is forwarded into the inhalation position when the inhaler is upright. The inhalation capsule is cut on the top and the bottom with stainless steel blades when turning the mouthpiece; the powder is then available for inhalation.	

Different lactose qualities in combination with budesonide (as a hydrophobic model drug) and salbutamol sulphate (as a hydrophilic model drug) were screened for their potential use with the DPI. The suitability of the lactose quality for potential use was verified by content uniformity testing and analysis of the aerodynamic particle size of the released aerosol.	

Lactose was purchased from three different manufacturers (Table I). Micronized budesonide (Farmabios srl, Italy) and salbutamol sulphate ([SBS] AstraZeneca, Germany) were used as model drugs; that is, drugs used as an example, although a variety of drugs could have been used. Methanol and acetonitrile were of chromatography quality (Karl Roth GmbH, Germany). 		

Table I:  Lactose and drug physicochemical characteristics.				

1-heptanesulphonic acid was purchased from Sigma Aldrich (Germany); the water used had been distilled twice.	

Particle size distribution. The particle size distribution of the drugs and the lactose was determined with a laser diffractometer (Sympatec GmbH, Germany). The powders were measured after dispersion by compressed air (2 bars).				

Specific surface area. The specific surface area was measured using the gas adsorption method with a Gemini 2360 (Micromeritics GmbH, Germany). The sample preparation was done according to a fixed protocol for 12 h under vacuum at 35 Â°C (VacPrep 061; Micromeritics GmbH).	

Particle roughness. The particle roughness was estimated by using an image analysis system (Leco 2001; Leco Instumente GmbH, Germany). The lactose particles were visualized with an optical microscope. Particle dimensions were evaluated using the Leco 2001 software and the roughness was calculated according to Equation 1.

Powder density. The density was determined with a helium pycnometer (Accupyc 1330; Micromeritics GmbH).	

SEM. Scanning electron microscopy (SEM) pictures were taken using a Philips XL 20 (Philips, The Netherlands). Samples were fixed on an aluminium sample holder with conductive double-sided adhesive tape (Leit-Tabs; Plano GmbH, Germany) and coated with gold in an argon atmosphere (50 Pa) at 50 mA for 50 s (Sputter Coater; Bal-Tec AG, Liechtenstein).	

HPLC. A mixture of 75% methanol and 25% water (V/V) was used as a solvent for the budesonide samples. The content of budesonide was determined using a validated high-performance liquid chromatography (HPLC) method using an RP18 (LiChroCART 125-4, LiChrospher 100; Merck, Germany) column and a mixture of 45% acetonitrile and 55% water as the mobile phase. Detection was performed at 243 nm.	

Water was used as the solvent for the salbutamol sulphate assay. The content of SBS was also determined by reverse-phase (RP)-HPLC. A mobile phase consisting of phosphate buffer, methanol, water and 1-heptanesulphonic acid was used. Detection was performed at 280 nm.	

Preparation of the powder blends. The powder blends (1.1% m/m drug content) were manufactured according to a fixed protocol: 	

testing for homogeneity by analysing 10 random samples (20 mg of powder per sample). 

Powder blends were accepted as homogeneous and used for further testing when the relative standard deviation (RSD) was less than 3%.	

Dose content uniformity. Size 4 hydroxypropyl methylcellulose (HPMC) capsules (Shionogi Europe BV, The Netherlands) were filled with exactly 20.0 mg of the powder blend corresponding to a drug content of 220 Âµg/capsule. The drug content of each delivered dose was assessed with the dose uniformity sampling apparatus as described in the European Pharmacopoeia (Ph. Eur.).

 The pressure drop across the device was adjusted to 4 kPa, which resulted in an air flow rate of 30 L/min through the device. 10 single doses were analysed per batch.		

Fine particle fraction. The aerodynamic particle size distribution was determined with an MLI multistage liquid impinger (Erweka Appartebau GmbH, Germany) at an air flow rate of 30 L/min through the device. Drug deposition on the single stages of the impinger was chemically assayed and the fine particle fraction was calculated as the amount of particles <5 Âµm.	

Physicochemical properties. The physicochemical characteristics and differences of the lactose are summarized in Table I. Inhalac 70 exhibited the largest average particle size (350%550% undersize) of 212 Âµm and the smallest BET surface area (Figure 2(b), Table I), whereas Inhalac 120 (Figure 2(a)) had a smaller average particle size (150 Âµm). Respitose 325 M and Lactohale 200 showed the smallest average particle size (350%: 60.6 Âµm and 83.0 Âµm, respectively [Figures 2(c) and (e) and Table I]), whereas Lactohale 100 and Pharmatose 110M had average particle sizes of 128.2 and 121.6 Âµm, respectively (Figure 2). 			

Table II:  Number of doses needed to empty the powder from the FlowCaps inhaler (average of 30 capsules) and Figure 2:  Characterization of the lactose qualities using laser diffraction: (a) Inhalac 120, (b) Inhalac 70, (c) Lactohale 200, (d) Lactohale 100, (e) Pharmatose 325, (f) Pharmatose 110.				

The differences in true density and surface roughness were only marginal and not significantly different, as expected for large crystals of the same material. The specific surface area varied between 0.07 m

/g (Lactohale 200), which was in relation to the particle sizes of the lactose qualities.	

Small differences in the surface texture and crystal form of the lactose particles could be seen on the SEM photographs (Figure 3). The Inhalac 70 consisted of agglomerated smaller lactose particles with minimal fines content; Inhalac 120 had a low fine content but showed more regular-shaped crystals, as did the other entities. Pharmatose and Lactohale contained small lactose particles as well. 		

Figure 3:  SEM photographs taken from the analysed lactose: (a) Inhalac 70, (b) Inhalac 120, (c) Pharmatose 110M. (Chart continued below)	

It should be noted that all lactose compounds under investigation can be used in dry powder inhalation formulations with respect to their shape, size and size distribution. Although they differed in particle size distribution, average particle size and fines content, all appeared capable of passing the cut capsule slit and were, therefore, used for further testing.	

Figure 3 continued:  (d) Pharmatose 325 M, (e) Lactohale 100, (f) Lactohale 200 (continued again below).	

Delivery of inhalation powder. The manufactured powder blends filled size 4 HPMC capsules and were emptied with the official sampling apparatus of the Ph.Eur. Initially, the emptying behaviour of the capsules upon inhalation was observed visually. Four of the selected lactose entities proved to be inappropriate for use with the FlowCaps inhaler as the capsules did not empty reproducibly (Table II). Pharmatose 110, Lactose 100, Inhalac 70 and 120 needed 10 or more actuations for complete powder delivery from the capsule. These lactose compounds had an average particle size of >100 Âµm, which are too large and exhibit too high a degree of mass inertia to be entrained by the air. Although the screened lactose compounds were unsuitable for use with the FlowCaps  DPI, they may work with other types of DPIs.	

Reproducible delivery of the powder from the capsules was possible with fewer than three inhalations using Pharmatose 325M and Lactohale 200 and, therefore, only these were used in further studies to assess the content uniformity of the delivered dose and the fine particle fraction of the powdered medication.	

Content uniformity. The results of the content uniformity tests are summarized in Figure 4. The measured delivered dose was normalized to the average of the 10 single delivered doses (=100%) and the limits of ±25% and ±35% of the average delivered dose were calculated. According to Ph.Eur., 9 out of 10 single doses must lie between ±25% of the average and 1 dose is allowed to fall between ±25% and ±35% of the average delivered dose.

 Values outside ±35% are only tolerable in justified cases.

 The US regulatory authorities demand tighter limits for the content uniformity, specifically ±20% and ±25% of the labelled dose.

Table III:  Aerodynamic properties of the investigated drug/lactose blends.	

As shown in Figure 4(a) and (b), the powder blends with salbutamol sulphate easily match the set criteria for content uniformity according to both Ph.Eur. and the US Food and Drug Administration (FDA) requirements; the doses were delivered consistently and were close to regulatory requirements.	

Content uniformity of the budesonide mixes exhibited different behaviour. Higher variations in the delivered dose, together with higher device retention, was observed. Accordingly, the tested blends did not comply with either FDA or Ph.Eur. standards. The particle size distributions of the salbutamol sulphate and budesonide used in this study were very similar, and the drugs were used in the same concentration (drug:lactose ratio 1:90.9). This effect is attributed to the different surface energies of the hydrophilic drug SBS and the hydrophobic drug budesonide, resulting in different blend properties (Figure 3(g) and 3(h)). 	

SBS is more uniformly distributed on the lactose surface with few agglomerates visible, whereas it exhibits higher cohesion in the budesonide and, accordingly, a higher agglomeration tendency. This in turn leads to poorer flowability and dispersibility of the budesonide blend and increased variations in the delivered dose. Accordingly, a formulation with a budesonide/lactose blend needs to be more thoroughly developed with respect to the budesonide size distribution and the lactose quality. There were almost no differences in the dose uniformity patterns between the two types of lactose used and they principally show the same trend with respect to the drugs.	

Figure 3 (continued):  (g) Budesonide/Lactohale 200 blend, (h) Budesonide/Pharmatose 325 M blend, (i) SBS/Lactohale 200 blend, (j) SBS/Pharmatose 325 M blend.	

Fine particle fraction. The results of the aerodynamic testing are summarized in Table III and Figure 5. Again, large differences in the performance of the powder blends was detected between salbutamol sulphate and budesonide. In addition, the aerosolization behaviour of the blends was shown to be dependent upon the lactose quality used.		

Dry powder inhalers are a well-accepted dosage form for pulmonary drug delivery and a wide variety are either currently available or in development. This article examines a premetered, capsule-based multidose inhaler for which different qualities of a-lactose monohydrate were screened.